This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The two Ca2+-dependent cysteine proteases, [unreadable]- and [unreadable]-calpain, are involved in various Ca2+-linked signal pathways, but differ markedly in their Ca2+ requirements for activation. Using diffraction data collected at CHESS, we have determined the structure of a [unreadable]-like calpain, which has 85% [unreadable]-calpain sequence (the first 48 and the last 62 residues of the large subunit are those from m-calpain) and a low Ca2+ requirement. This construct was used because [unreadable]-calpain itself is too poorly expressed. The structure of [unreadable]-like calpain is very similar in overall fold to that of m-calpain as expected, but differs significantly in two aspects. In comparison with m-calpain, the catalytic triad residues in [unreadable]-like calpain, His and Cys, are much closer together in the absence of Ca2+;and significant portions of the Ca2+-binding EF-hand motifs are disordered and more flexible. These structural differences imply that Ca2+-free [unreadable]-calpain may represent a partially activated structure, requiring lower Ca2+ concentration to trigger its activation